510 PROTOPLASM 



control metabolic rate has long been known. Now we know the 

 exact molecular structure of the substance thyroxin, which is 

 responsible for these effects. 



We are here interested primarily in the secretions of cells other 

 than those which are aggregated into specialized tissues or glands. 

 Among the first of these to be studied was the "organizing sub- 

 stance" isolated from embryos by Spemann. 



Chemical Embryology. — Among growth-regulating substances 

 are those responsible for the proper development or organization 

 of an embryo. Why an embryo grows into this or that kind of 

 animal is determined by its chromosomes, but for normal develop- 

 ment certain growth regulators or organizers are necessary. The 

 first definite proof of the presence of such substances came from 

 the work of Spemann, who, in 1918, found that the region of the 

 dorsal lip of the blastopore is an Organizator, or differentiator. 

 On grafting a piece of it to another embryo of different species, 

 he found that it caused cells, which would otherwise have gone 

 along quite other lines of growth, to develop into a secondary 

 embryo. The organizing property is not characteristic of the 

 tissue as such (of the blastopore lip) ; other tissue will do as well 

 if it has been in contact with the organizer. Obviously, some 

 chemical substance is responsible. Further, the regulating sub- 

 stance is not specific; that from any species (so far as tried) will 

 serve to stimulate and direct the cells of another species. 



The next task in the work was the isolation of the organizing 

 substance and a determination of its chemical nature. J. Need- 

 ham has been responsible for a great deal of this work, though 

 others have contributed much. Mangold found that the organ- 

 izer retains its activity unimpaired for a long time. It may be 

 boiled, narcotized, crushed, dried, or frozen without loss of its 

 inductive power. It appears definitely to be an ether-soluble 

 substance, possibly lipoidal in nature; it may be a sterol or a 

 hydrocarbon ; if the latter, it is comparable to the cancer-produc- 

 ing hydrocarbons. Certain other substances, however, function 

 as organizers, e.g., thyroxin. 



Plant Hormones. — Plant-regulating and growth-stimulating 

 substances were first referred to by Sachs and later more exten- 

 sively studied by Haberlandt, who was particularly interested in 

 wound-healing hormones. The healing of wounds in animals as 

 well as in plants and the regeneration of body parts {e.g., in lower 



